I've seen my dad's, brother's and my younger cousin's throughout my life.
Seen my dad (flaccid) when he was changing or coming out the shower, etc. when I was younger. I would say that now I'm older, flaccid wise, I'm bigger than he is. Never seen him erect. He is cut, whereas I'm uncut.
I have seen my brother's both flaccid and erect whenever we were changing, in the bathroom, etc. He's looks around the same size as me erect but smaller when flaccid. He has more of a mushroom head whereas mine's is more bell shaped. He's also uncut.
Me and my younger cousin were brought up around each other so have seen each other naked many times. Ive seen him flaccid and erect. Last time I seen him erect was probably when we were both 14/15 and at relatives houses. We've both seen each other flaccid when sharing rooms at xmas time, etc for family holidays. He's also uncut.